Good question. Interrail connect with a number of railway companies’ systems to book reservations. I'm not sure which system Interrail use to book these trains but normally, as soon as that system has the trains available, Interrail should also be able to book them.

But if SNCF can already book these trains, then you should be able to book it via SNCF, at a ticket office or via phone. That way, you won't have to pay Interrail's booking fee.

As is often mentioned both the Interrail web planner and the app planner are only updated every so often, and then independently.

To complicate it further operators also often delay sending their timetables to the independent planners so the advice is always to check with the operators own timetables, especially for after the major multi operator timetable revision of Dec 12.

As for reservations there are plenty of guides for where best to source reservations for each provider on the community, but it is generally recommended that you do not rely on the IR reservation service, not least because there are limited operators they have access to reservations and then they may be limited (inc TGVs), and they charge an extra 2 euro per reservation. I believe cross border TGV services are either not available on the IR res tool or difficult to get at the lower price for TGVs in general whilst pass quotas are available,

The cross-border TGVs are available via Interrail. It's the domestic reservations in France that are currently only available for 20€ via Interrail, also when 10€ quotas would be available.

Hint-only for non-Swiss who look at the frankeli to spend: for cross-border the surcharge is high, very high (in a non-Schwyz eye) and it can easily be lowered to domestic by cross-border-till MUlhouse, maybe Strasbourg, on the TER trains.

It may even be the case-if this your only trip that day, that book now=advance is cheaper as Passday+surcharge.

Thanks all for the replies, I called SNCF and they could make the reservation directly however as @mcadv notes - the price with the interrail pass in my case was the same as buying a ticket so not really worth wasting a domestic travel day.

The cross border TGV are available to book via other means earlier than making reservations on the same trains to board in france.  I’ll wait and see if seats are availble at the €20 or €10 interrail to board in Bellegarde once booking is permitted (for me it’s the same distance from home)

Although the TGV-Lyria seems expensive for cross-border journeys, for journeys on the same train within France it seems to be priced at the same interrail rates as the regular TGV.

Yes TGV Lyria is the biggest scam for Interrail/Eurail users. 

Whinin Switzerland you don even need a reservation for the TGV. In France 10€/20€ and for the small "international" they whant so much.

Calling it a scam is a bit unfair. This is called business. They so or so get their trains fully-booked, so they think economically and sell the reservations for a high price because they could so or so sell the seat for a higher price than an Interrail user would pay… 
Or you think with the old 10 euro reservation fee on TGV Lyria they got the same money like from a normal ticket buyer + the share of revenue they get from EURAIL? Never ever…

Interrailers who do their homework don’t have to pay this high fee and can still get in a good time to Paris.

For me it is a scam. To pay more than on Eurostar or the RENFE-SNCF services. 2nd class is ok, but 1st class about 70€ is really a scam. 

Paris-Zürich I used back in 2019 ICE to Strasbourg and from there TER to Basel and IC to Zürich. 

Unfortunately in life there is nothing to be gained from “the good old days”.

Whether we like it or not the new world of reservations for pass users is upon us.

Also the profile of IR users is clearly changing. As well as backpackers and students, now families and mature couples are using them for new adventures, and when you have a budget of £1000 or more for your trip a few euros on reservations seems good value for access to these modern trains. 

Just like low cost airlines, the operators run sophisticated dynamic pricing software for normal passengers - busy trains cost more, quiet trains are cheap.

For pass users they cannot apply that process - although SNCF do have a 2 tier pricing process. I assume that the res fees for pass users are based on some sort of calculation on what the average variation on retail prices is and balancing the number of pass users they have space for.

If we don’t want to pay these premiums then we simply plan for the slower trains or look for the very good advice on minimising them from the experienced community members.

The ONLY operator within eurail/interrail to charge more than €10 for reservations is SNCF (and SNCF controlled subsidiaries), along with the swinging quotas it is basically a scam. Let interrail sell the passes with vague notices about reservation fees then SNCF hit them up with huge penalty rates on these services.

If other operators started following this model then interrail as a product would be dead, there would be no way any normal tourist would get value out of passes versus normal tickets, as it is many now do not and the stock answer told to people inquiring on the likes of Tripadvisor is don’t use interrail/eurail.

The “Just use slower trains” line is a well used one but the reality is that this is increasingly difficult, particularly in countries that have gutted these services in favour of High-speed trains, one of the worst for this: SNCF.